It is known in the prior art to control the movement of one or more vehicles coupled together to form a train through a fixed block track circuit signaling system. Specific signal blocks of track are established by predetermined very low impedance electrical signal boundaries provided at the ends of each signal block. When a train vehicle is present in a given signal block, at least one vehicle axle of the train electrically shorts between the two conductive track rails on which the vehicle runs. A signal transmitter is coupled to the track at one end of each signal block and a cooperative signal receiver is coupled to the track at the opposite end of that signal block for providing desired control of the train movement and detecting the occupancy of a train vehicle within that signal block. The train position is detected electrically as the individual vehicles of the train move along the track rails, passing through succeeding blocks, as described in U.S. Pat. RE No. 27,472 of G. M. Thorne-Booth and as described in an article published in the Westinghouse Engineer for September, 1972 at pages 145 to 151 by R. C. Hoyler.
The vehicle detection equipment is located at the wayside of the track, and when a train vehicle is detected in a given signal block, a control signal is provided to influence the speed code of the next previous signal block and if desired this control signal can ripple back to one or more previous signal blocks. Under certain abnormal operational conditions, such as when electric power to the train vehicle is lost or when a substantial corrosion film or ice builds up on the top of the track rail members, there is a small possibility that the conventional signaling system may not detect a train vehicle occupancy within a given signal block. The conventional signaling system detects the train vehicle shunt impedance, and if this is abnormally high for some reason the train vehicle occupancy detection becomes more difficult. An excessive corrosion film will increase the train vehicle shunt impedance by forming a barrier layer to make more difficult the detection of the train vehicle occupancy.
The need for sensing train vehicle presence in a given present signal block has led to occupancy detection and sequential occupancy release control of train movement, such that when a vehicle occupancy is detected in a present signal block it is necessary to subsequently detect occupancy in the next succeeding signal block before a release is desired of the occupancy behind the vehicle. The occupancy in a previous signal block is retained and not released until the train vehicle occupancy is positively detailed in the next succeeding signal block.
It is known in the prior art as generally described at page 51 of Business Week magazine for Mar. 2, 1974 to determine the occupancy detection and sequential occupancy release control of a train vehicle moving in relation to successive signal blocks N- 2, N- 1, N, N+ 1, and N+ 2 and so forth by utilizing the following train movement control algorithms: EQU Set Q.sub.N = Q.sub.N.sub.-1 Q.sub.N.sub.+1 I.sub.N.sub.+1 B.sub.N.sub.+1 +Q.sub.N.sub.+2 I.sub.N.sub.+2 B.sub.N.sub.+ 2 +Q.sub.N.sub.+3 . . . , etc. (1). EQU Reset Q.sub.N = Q.sub.N.sub.+1 ( 2). EQU set B.sub.N = I.sub.N Q.sub.N Q.sub.N.sub.-1 Q.sub.N.sub.-2 B.sub.N.sub.- 1 +Q.sub.N.sub.-3 B.sub.N.sub.- 2 +Q.sub.N.sub.-4 . . . , etc. (3). EQU Reset B.sub.N = I.sub.N ( 4). EQU o.sub.n = i.sub.n + q.sub.n ( 5).
where I.sub.N is a primary train vehicle occupancy indication signal for a typical signal block N, Q.sub.N is a back up protection signal for signal block N, B.sub.N is a pseudo or false occupancy indication signal for block N and O.sub.N is the occupancy control signal operative with the primary train control system for signal block N. The O.sub.N signal controls the movement of a subsequent train vehicle in relation to previous signal block N-1 when a train vehicle is detected within present signal block N. The above set Q.sub.N equation (1) is operative with the indicated well known AND and OR logic relationships to set the signal Q.sub.N to true when the signal Q.sub.N.sub.-1 is true and the not signal Q.sub.N.sub.+1 is true and the signal I.sub.N.sub.+1 is true and the not signal B.sub.N.sub.+1 is true. In addition, the OR operation of the above said Q.sub.N equation (1) is operative to set the signal Q.sub.N to true when the not signal B.sub.N.sub.-1 is a false, if the signal Q.sub.N.sub.-1 is true and the not signal Q.sub.N.sub.+1 is a true and the signal I.sub.N.sub.+1 is a true and the not signal Q.sub.N.sub.+2 is true and the signal I.sub.N.sub.+2 is true and the not signal B.sub.N.sub.+2 is true, and so forth for all of the remaining track signal blocks ahead of block N, up to a theoretical infinite number of signal blocks. In general a false signal has a zero volts value and a true signal has a predetermined volts value. The above reset Q.sub.N equation (2) is operative to reset the signal Q.sub.N to a false when the signal Q.sub.N.sub.+1 is true. The above set B.sub.N equation (3) is operative with well known logic relationships to set the signal B.sub.N to true when the occupancy indication signal I.sub.N is true and the not signal Q.sub.N is true and the not signal Q.sub.N.sub.-1 is true and the not signal Q.sub.N.sub.-2 is true and the not signal B.sub.N.sub.-1 is true. In addition the OR operation of the above said B.sub.N equation (3) is operative to set signal B.sub.N to true if not signal B.sub.N.sub.-1 is false, when signal I.sub.N is true and the not signal Q.sub.N is true and the not signal Q.sub.N.sub.-1 is true and the not signal Q.sub.N.sub.-2 is true and the not signal Q.sub.N.sub.-3 is true and the not signal B.sub.N.sub.-2 is true, and so forth for all of the remaining track signal blocks subsequent to block N up to a theoretical infinite number of signal blocks. The above reset B.sub.N equation (4) is operative to reset signal B.sub.N to false when the not signal condition I.sub.N is true. The above said set O.sub.N equation (5) is operative to set the occupancy control signal O.sub.N to true when the signal I.sub.N is true or the signal Q.sub.N is true.